[WIP] Add target features

[anderleich]

ATTENTION! I'm opening a new PR to add target features only.
The previous PR was #2289. Closed

This PR intends to add target features support to OpenNMT-py. Target features are appended to the textual data. It uses the special character ￨ as a feature separator, as in the previous versions of the OpenNMT framework.

 I￨1￨3 love￨0￨1 eating￨0￨1 pizza￨0￨1

Target features can be used along source features. A YAML configuration file would look like this:

data:
    train:
        path_src: src_with_features.txt  #  I￨1￨3 love￨0￨1 eating￨0￨1 pizza￨0￨1
        path_tgt: tgt_with_features.txt  #  Me￨1 gusta￨0 comer￨0 pizza￨0
        transforms: [onmt_tokenize, inferfeats, filtertoolong]
    valid:
        path_src: src_with_features.txt
        path_tgt: tgt_with_features.txt
        transforms: [onmt_tokenize, inferfeats]

save_data: ./data
n_sample: -1

# # Vocab opts
src_vocab: data.vocab.src
tgt_vocab: data.vocab.tgt
n_src_feats: 2
n_tgt_feats: 1
src_feats_defaults: "0￨1"
tgt_feats_defaults: "1"
feat_merge: "sum"

@vince62s for the moment I've just copied all the necessary changes from #2289.

[yash-srivastava19]

Is this PR still WIP? Any update when this will be done? Or if you need any help in this PR? I might be of some use?

[vince62s]

not sure where @anderleich left this on hold.